a) Field of the Invention
The present invention relates to an objective lens system for endoscopes using a graded refractive index lens component.
b) Description of the Prior Art
There are known many objective lens systems having compositions exemplified by that of the objective lens system disclosed by Japanese Patent Kokai Publication No. Sho 60-46,410 illustrated in FIG. 1. This system is of the retrofocus type which consists of a diverging lens unit (a front lens unit) and a converging lens unit (a rear lens unit) which are disposed before and after respectively an aperture stop.
In the lens system having this composition, the diverging front lens unit has a function to Widen a field angle of the lens system and a role to reduce a Petzval's sum, whereas refractive powers are shared among many lens elements for producing little aberrations in the converging rear lens unit. For this reason, the rear lens unit is composed of three positive lens elements and a cemented lens component. Aberrations are corrected favorably in the objective lens system as a whole by adopting the power distribution described above.
However, this objective lens system comprises a large number of lens elements and has a long total length. Even when the number of lens elements are reduced by using an aspherical surface in this objective lens system, chromatic aberration cannot be corrected favorably though Seidel's aberrations (spherical aberration, astigmatism and coma) and the Petzval's sum can be reduced.
The retrofocus type objective lens system for endoscopes consists of the diverging front lens unit and the converging rear lens unit which are disposed on both sides of the aperture stop as described above, and the front lens unit consists in most cases of a single lens element which has a planar surface on the object side and a concave surface on the image side. The front lens unit has a negative refractive power and, since the lens unit is disposed on the object side of the aperture stop, the front lens unit produces negative lateral chromatic aberration. Since most of the lens elements disposed in the converging rear lens unit have positive refractive powers, the rear lens unit produces remarkable negative lateral chromatic aberration.
In order to correct the remarkable negative lateral chromatic aberration which are produced by the front lens unit and the rear lens unit as described above, it is conventional to use a cemented lens component which consists of a convex lens element made of a glass material having a high dispersing power and a concave lens element made of a glass material having a high dispersing power in the rear lens unit. Further, the lens elements disposed in the converging rear lens unit are made of glass materials which have relatively high refractive indices and relatively weak dispersing powers so that the lens surfaces disposed in the rear lens unit produce lateral chromatic aberration as little as possible.
The objective lens system disclosed by Japanese Patent Kokoku Publication No. Sho 60-46,410 has a wide field angle and faborably corrected aberrations. However, this objective lens system comprises lens elements as many as six, has a complicated composition and requires high cost for manufacturing. This objective lens system has defects that it has a long total length and that, when the objective lens system is used in medical endoscope, it prolongs a hard section of the endoscope which is to be inserted into human bodies, thereby making it impossible to inspect interiors of curved organs.
As a conventional example of an objective lens system for endoscopes using a graded refractive index lens component, there is known the lens system which is disclosed by Japanese Patent Kokoku Publication No. Sho 47-28,061. This lens system has planar surfaces on both sides and uses a graded refractive index lens component which has a refractive index distribution in the radial direction. This objective lens system has a simple composition which is desirable for use with endoscopes, but cannot have a wide field angle which is determined dependently on a distribution of refractive powers therein, has an outside diameter which is large at a certain degree, produces remarkable offaxial aberrations at high image heights, and cannot be used in endoscopes having large outside diameters.